The present invention relates to a helicopter hazardous ground object warning system and especially to a warning system for alerting a helicopter pilot during hover, takeoff or landing that the helicopter's main rotor blade, tail rotor blade, and/or fuselage are approaching a ground based object which would be hazardous to the helicopter in a collision and for alerting ground personnel who may be walking in the near vicinity of a landed helicopter along a path which intersects or comes close to an operating rotor.
Accident studies have indicated that a high percentage of helicopter accidents during hover, takeoff and landing involve collisions between the tail or main rotor blades and ground based objects that were not seen or were neglected or forgotten by the pilot because of other pilot distractions at the time of the accident. These studies also indicate that the primary cause of ground personnel accidents is ground personnel walking into moving main or tail rotor blades. In addition, helicopters have had the problem of hitting electrical or communication wires which are difficult to see by a low-flying helicopter and can result in the loss of the helicopter and crew by the helicopter colliding with pylons, radio masts, and cables during either actual missions, training exercises or the like when the helicopter is flying at low altitudes. This is especially true in the case of military use of helicopters, such as in combat, rescue missions, and the like, in which the helicopters are flying at low altitudes.
A considerable effort has been expended in solving the problem of low flying helicopters hitting wires. Prior techniques have been used which include microwave and millimeter wave radar systems, an infrared based radar system employing a carbon dioxide laser and a passive system which detects the magnetic flux from a live transmission line. Each of these prior techniques has certain inherent drawbacks which make them unsatisfactory in some circumstances. The microwave and millimeter wave radar systems are characterized by long wavelengths and, because of the specular reflection of metal surfaces at these wavelengths, wires and cables are difficult to detect. Also, emission from microwave and millimeter radar transmitters is propagated over large distances and over a broad area which causes much ground clutter. A carbon dioxide laser radar, which operates in the mid-infrared region at a wavelength of 10.6 micrometers, is capable of detecting small obstacles. However, the laser radar requires a scanning mirror system which deflects the transmitted stream of laser pulses over a large field-of-view to intercept an object, such as a cable, and a special laser detector for detection of the reflected laser pulses. The magnetic flux detection system is useful only within ranges too short for obstacle avoidance so that cables purposely deployed across valleys and the like to intercept helicopters cannot be detected using a magnetic flux system.
Finally, it has been suggested to use a wire obstacle avoidance system for helicopters, such as taught in the Koechner, U.S. Pat. No. 4,902,126, which uses a solid state laser transmitter for emitting radiation in the near-infrared wavelength region and uses a beam dividing device or a plurality of laser diode arrays for generating separate laser beam lobes which are passed through an objective lens for deflection in slightly different directions to define a field of coverage in front of the helicopter. A wire obstacle in the field of coverage intercepts one or more of the lobes and reflects signals to a receiver detector array which compares the time of the return signal with the time of transmitted laser lobes with the difference therebetween being a function of the range between the obstacle and the helicopter. The range information can then be displayed to the pilot in time to take evasive action.
There is also a wide variety of patents describing systems which are used for automatically detecting the presence or absence of one land vehicle in front of another. There are also several patents for obstacle sensors, such as shown in the Wada et al., U.S. Pat. No. 5,076,384, for an ultrasonic obstacle sensor. A number of prior patents deal with automatically guided vehicles in warehouses and the like, and some of these, such as the Field, U.S. Pat. No. 5,020,620, use a laser guided system.
In contrast to these prior systems, the present helicopter hazardous ground object warning system is not only suitable for recognizing wires but is designed specifically to avoid hazardous ground objects which can completely surround the helicopter when the helicopter is hovering or is taking off or landing. These obstacles might be in line with the main rotor blade, the tail rotor blade, or the fuselage so that a pilot warning can prevent a collision. Thus, the helicopter warning system of the present invention can be used for the tail rotor blade and tail of the vehicle covering a plane of (plus or minus) 180.degree. or 360.degree. around the bottom of the fuselage of the helicopter. The minimum range can be set such that helicopter components, such as the landing skids, will be blanked out as the sweeping laser beam rotates thereby. Also, a second feature of the subject invention is the warning it can provide to ground personnel walking in the vicinity of a landed helicopter with main and tail rotor blades still operating thereby preventing them from walking into the rotors.